The invention pertains to methods of forming a plurality of spheres, and also pertains to pluralities of spheres.
Numerous applications exist in which it is desired to form pluralities of spheres which are uniform in diameter, spherical shape, and surface properties, with exemplary applications including formation of glass and/or ceramic beads for column-packing materials; formation of lead, bismuth, tungsten or steel beads for birdshot; and formation of solder beads (or balls) for utilization in semiconductor processing applications.
The formation of solder beads for semiconductor processing applications can be particularly difficult, as the uniformity requirements of such beads can be exceptionally high. For instance, an exemplary semiconductor processing application utilizing solder beads is so-called ball grid array processing wherein solder beads are transferred to a semiconductor device (or package) and subsequently melted to form electrical interconnections relative to the device. The beads are formed into a desired array by rolling the beads across a template having a plurality of indentations therein, and subsequently shaking the template to remove excess beads while leaving beads remaining in the indentations. If the beads are not all entirely uniform, some of the beads will roll differently than others, which can lead to some indentations not being filled and/or to some excess beads not rolling entirely from the template. Ultimately, the beads remaining on the template after the shaking are lifted (typically by a vacuum device) and transferred to a semiconductor substrate whereupon the beads are supposed to form a pattern defined by the pattern of indentations in the template. However, if excess beads remain on the template after the shaking, or if some of the indentations were not filled in the template, the pattern of beads transferred to the semiconductor substrate will be wrong, which can lead to failure of devices associated with the semiconductor substrate. The described ball grid array process is an important process relative to formation of electrical interconnects during chip-scale packaging of semiconductor devices, and can be associated with, for example, flip-chip applications. Additionally, the ball grid array process can be utilized during formation of packages larger than chip-scale.
Several methods have been developed for forming pluralities of small spheres, including methods in which the spheres are punched, cut, and/or rolled from starting materials. However, there remains a need for improved methodology of forming pluralities of uniform spheres.
In one aspect, the invention encompasses a method of forming a plurality of spheres. A passageway is provided, and the passageway terminates in an orifice. A liquid is flowed through the passageway and expelled through the orifice to form drops. The drops are then passed through a fluid to cool and solidify the drops into a plurality of spheres. At least some of the spheres are collected. A pressure of the liquid is maintained within about xc2x110% of a value during the formation of the drops that are cooled into the collected spheres. In particular embodiments, the pressure of the liquid is maintained steady to within about xc2x13% during the formation of the drops that are cooled into the collected spheres, and can, for example, be maintained steady to within about xc2x10.05% during the formation of the drops that are cooled into the collected spheres.
In another aspect, the invention encompasses a plurality of at least several hundred spheres having a diameter of less than about 0.05 inches and characterized by at least 95% of the spheres being within about xc2x11.2% of a mean diameter of the spheres.